Oxygen Availability Sends Two Soil Carbon Substrates on Divergent Fates

A five-month soil experiment has revealed that the fate of carbon buried beneath our feet depends on a surprisingly specific combination of chemistry and oxygen. Glucose, a sugar that microbes can readily consume for energy, was retained in soil far more effectively than oxalate whenever oxygen was available—even when that oxygen arrived only intermittently. But under permanently oxygen-free conditions, the advantage disappeared: oxalate became just as effective as glucose at entering solid soil-carbon pools, while microbial respiration largely stalled. The findings, reported by Fiona M. Ellsworth and Richard E. Marinos in Biogeochemistry, challenge the idea that a molecule’s ability to stick directly to minerals is always the dominant predictor of whether it will remain in soil. Instead, the results show that microbial energy metabolism can govern carbon storage under both stable and fluctuating oxygen conditions, while mineral chemistry becomes more important when anaerobic conditions persist.

The distinction matters because soils hold more carbon than the atmosphere and vegetation combined, much of it in mineral-associated organic matter, or MAOM. This carbon can remain protected for centuries to millennia when organic molecules become attached to clay minerals, iron compounds and other reactive surfaces. Yet soil is not chemically static. Rainfall, flooding, drainage, root activity and microbial respiration can repeatedly switch microscopic environments between oxygen-rich and oxygen-poor states. Those changes alter the oxidation state of iron, the solubility of minerals and the ability of microorganisms to break down organic compounds. When iron-bearing minerals are reduced, they may dissolve and release previously protected carbon. When oxygen returns, minerals can re-form and potentially capture carbon again. The new experiment tested whether these redox oscillations—the chemical equivalent of repeatedly flipping an oxygen switch—interact differently with carbon molecules that have contrasting properties.

The researchers chose glucose and oxalate because they represent two common forms of low-molecular-weight carbon released by plants and roots. Glucose is relatively chemically reduced and yields substantial energy when microorganisms oxidize it to carbon dioxide. Oxalate, an organic acid with two carboxyl groups, is more oxidized and provides less energy during further oxidation, but it has a strong affinity for binding to soil minerals. In simplified terms, glucose is attractive food for microbes but a relatively weak mineral glue, whereas oxalate is less rewarding food but a powerful competitor for mineral binding sites. The study therefore set up a direct test of two possible routes to long-term soil-carbon storage: microbial processing followed by incorporation into organic matter, or direct sorption of an intact molecule onto mineral surfaces.

For the experiment, the scientists collected silty clay loam from the A and upper B horizons of a mixed deciduous forest soil at Margery Gallogly Nature Preserve on Grand Island, New York. The soil contained 4.8 percent organic matter and substantial free iron, providing the clay and iron-rich surfaces needed for mineral-organic interactions. They added either glucose or oxalate labeled with carbon-13, a non-radioactive isotope that allowed the researchers to distinguish newly added carbon from carbon already present in the soil. Each gram of dry-equivalent soil received 2 milligrams of labeled carbon—about 4 percent of the soil’s existing carbon content. The amended material was sealed in airtight glass jars and incubated under four regimes: continuously aerobic, oxygenated and oxygen-free on a weekly cycle, oxygenated and oxygen-free on a two-week cycle, or continuously anaerobic. Ten replicate jars were prepared for every substrate and oxygen combination, alongside unamended controls.

Over 20 weeks, the team repeatedly sampled gases in the jar headspaces, measuring carbon dioxide, methane and carbon dioxide containing the carbon-13 label. At the end of five months, the researchers separated the remaining soil carbon into chemically and physically distinct pools. A density separation divided relatively light particulate material from the heavier MAOM fraction. The heavy fraction was then subjected to sequential chemical extractions designed to identify carbon associated with polyvalent cation bridges, reducible minerals, non-reducible short-range-order minerals and crystalline mineral phases. Water extractions measured carbon that remained dissolved or readily mobilized. This approach did not simply ask how much carbon remained in the soil; it tracked where the added carbon went, whether it was respired as gas, dissolved in water, loosely retained in particulate matter or attached to mineral surfaces. Statistical comparisons used two-way analyses of variance, testing the independent and interactive effects of carbon substrate and redox regime.

The central result was stark. Under continuously aerobic conditions and under both intermittent oxygen treatments, about seven times more added glucose carbon was retained as soil organic carbon than added oxalate carbon. The difference appeared in both the light fraction and MAOM, with glucose retention averaging 7.4 times higher in the light fraction and 6.4 times higher across MAOM fractions. Oxygen did not need to be continuously present for this pattern to emerge. Weekly or biweekly anaerobic intervals did not significantly change the total amount of glucose or oxalate carbon stabilized compared with persistent aerobic incubation. The researchers interpret this as evidence that anaerobic periods temporarily pause microbial transformation and mineral-associated carbon formation, rather than creating a lasting storage advantage. Once oxygen returns, aerobic metabolism appears to dominate the overall carbon trajectory.

The explanation lies partly in microbial carbon-use efficiency, a measure of how much consumed carbon microorganisms convert into biomass rather than releasing as carbon dioxide. Oxygen is an energetically favorable terminal electron acceptor, enabling microbes to extract more energy from many substrates. Glucose, with its relatively high bioenergetic yield, can therefore support greater microbial growth under oxygenated conditions. Microbial cells and their residues are increasingly recognized as important sources of MAOM: organisms consume plant-derived compounds, build biomass and eventually leave behind chemically altered remains that bind to minerals. Oxalate follows a different path. Although it can bind strongly to minerals, its lower energy yield limits the microbial biomass produced per unit of carbon under aerobic conditions. In the experiment, much more oxalate remained dissolved in water—about five times more than glucose—and more was respired under continuously aerobic conditions. Its mineral-binding ability did not compensate for its weaker capacity to fuel microbial processing.

The picture changed completely in the permanently anaerobic jars. Respiration was strongly suppressed for both substrates, and much larger amounts remained in the aqueous phase: roughly 108 times more glucose carbon and 63 times more oxalate carbon remained dissolved than under the other redox regimes. Under these oxygen-free conditions, the difference in total solid-phase carbon between glucose and oxalate vanished. Oxalate-derived carbon entering MAOM increased compared with aerobic treatments, while glucose-derived carbon entering MAOM decreased. The researchers propose that suppressed microbial uptake under sustained anaerobiosis gave oxalate more time to associate directly with mineral surfaces. Because oxalate is a strong sorber, it may bind to clay or iron-containing minerals even when microbial transformation is energetically constrained. Glucose, by contrast, lost the microbial advantage that helped stabilize it under oxygenated conditions. The authors caution that isotope measurements cannot prove that labeled carbon recovered from MAOM remained chemically intact as oxalate or was converted into microbial residues, but direct sorption is a plausible explanation.

The experiment also exposed a potentially troubling side effect of oxalate. Under aerobic and fluctuating conditions, adding oxalate caused a strong priming effect: it stimulated the breakdown of carbon that had already been present in the soil. Compared with unamended controls, oxalate increased respiration of existing soil carbon by 1.8 times under persistent aerobic conditions and by about 1.9 times under the intermittent regimes. It also released far more pre-existing carbon into soil solution—9.1 times more under continuous oxygen, 24.7 times more under weekly cycling, 17.3 times more under biweekly cycling and 4.3 times more under persistent anaerobiosis. One likely mechanism is that oxalate’s small size and carboxyl groups allow it to displace older carbon from mineral surfaces, a process sometimes compared with molecular “unbuttoning” of mineral-organic associations. Once released into solution, that carbon becomes accessible to microbes. Fluctuating oxygen may intensify the process by dissolving iron-associated carbon during anaerobic intervals and exposing it to oxidation and microbial consumption when oxygen returns.

Glucose did not produce the expected positive priming effect. It neither increased respiration nor substantially increased solubilization of existing soil carbon under aerobic or fluctuating conditions, and it actually suppressed respiration of existing carbon under persistent anaerobiosis. The researchers suggest that microbes may have preferentially consumed the added glucose, reducing their need to attack older carbon. This behavior could be especially pronounced when oxygen and other electron acceptors are scarce. The results therefore complicate broad claims that adding labile carbon will universally accelerate the loss of stored soil carbon. The outcome depends on the molecular identity of the input, the minerals present, the availability of oxygen and the microbial community’s energetic constraints. A root exudate rich in organic acids may destabilize existing mineral-associated carbon even as it adds new carbon, while a sugar may be routed more efficiently through microbial biomass.

The study’s controlled jars cannot reproduce the full complexity of a living forest soil, where roots, fungi, fauna, water movement, temperature and nutrient availability interact over seasons and decades. The carbon addition was also deliberately high to ensure that the carbon-13 label could be recovered after five months. Even so, the findings offer a mechanistic warning for soil-carbon models and climate strategies: oxygen availability alone is not enough to predict whether new carbon will persist. Models must also account for the chemical identity of plant inputs and for the contrasting behavior of carbon compounds during microbial uptake, mineral sorption and priming. In particular, wetland soils, flooded agricultural fields, compacted soils and periodically saturated landscapes may respond differently depending on whether anaerobiosis is brief or sustained. The authors’ broader message is that soil carbon is not a single pool with a single fate. It is a moving network of molecules, microbes, minerals and redox reactions—and a small change in molecular structure can determine whether carbon is stored, dissolved, respired or used to unlock older reserves.